1. Field of the Invention
The invention relates to circuit protection devices for diodes, in particular, guard bands for Schottky diodes.
2. Prior Art
Metal-semiconductor diodes, often referred to as Schottky diodes, generally include a metal layer deposited on a n-type monocrystalline silicon body which includes an epitaxial layer. A substantial difference in barrier heights between the forward biased and reversed biased metal-semiconductor interface exist. When sufficient forward biased is applied this barrier is essentially removed, thereby allowing electrons to freely flow from the semiconductor material into the metal.
Schottky diodes provide efficient rectification in power applications since their forward voltage drop is less than that of a PN junction diode. Moreover, these devices have fast recovery times making them particularly useful for high frequency rectification.
In practice, the majority of failures of Schotty diodes are due to transient voltages. These failures occur since surface defects in the semiconductor material or defects in the metal-semiconductor interface cause current crowding at the defects which damages the semiconductor material. As will be seen, the present invention provides an integral PN junction (diode) in the form of a guard band. This junction diode when reversed biased, conducts before an avalanche breakdown occurs in the Schottky diode.
Guard bands are employed in the prior art with Schottky diodes. Typically, a p-type guard band is disposed in the surface of an n-type epitaxial layer around the metal-semiconductor barrier. These guard rings are used to reduce the edge leakage current effects. In such prior art guard bands, the breakdown voltage of the junction diode is larger than that of the Schottky diode.